Solothurn (Fr. Soleure), a N. W. canton of Switzerland, bordering on Basel Country, Aargau, and Bern; area, 303 sq. m.; pop. in 1870, 74,713, of whom 62,078 were Roman Catholics. The Jura mountains occupy a part of the canton, and the remainder of the surface is level and fertile. It is traversed by the river Aar, a tributary of the Rhine. Gold, silver, iron, and lignite are found. The soil is remarkably fertile. A great deal of the surface is occupied by meadows and pastures, upon which large numbers of cattle are kept. The forests are extensive, and afford valuable timber. German is the language of the canton. The government was formerly aristocratic, but democratic principles have been largely introduced into it, especially by the revision of the constitution in 1841. - Solothurn, the capital, is at the foot of the Weissenstein, on the Aar, 17 m. N. by E. of Bern; pop. in 1870, 7,054. It has one of the finest cathedrals of Switzerland, an arsenal with a large collection of ancient armor, and a museum containing a rich collection of Jura fossils.

Till 1874 it was the seat of the bishop of Basel.